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Abstract:

An off-period adjustment circuit 17 changes an off period of the switching
element 2 after a maximum on period according to duration of a period
from when the switching element 2 is turned on to when a current flowing
through the switching element 2 reaches an overcurrent detection level.
With this configuration, an oscillation frequency of the switching
element 2 is changed so that a switching power supply can provide output
energy independently of change in input voltage. Consequently provided is
a less costly switching power supply apparatus and a semiconductor device
used for the switching power supply apparatus, with which currents to
loads are made uniform among regulation devices without providing
different circuit constants according to a range of input voltages.

Claims:

1. A semiconductor device comprising a control circuit which controls a
switching power supply apparatus that converts an input direct current
voltage into a regulated output direct current voltage,wherein the
switching power supply apparatus includes an output voltage detection
circuit which detects a change in the output direct current voltage and
transmits, to said control circuit, a feedback signal so that said
control circuit controls switching operation which a switching element
performs by supplying and blocking an input direct current voltage,said
control circuit includes:an oscillator configured to determine a
switching oscillation frequency of said switching element;a feedback
control circuit which determines, according to the feedback signal
transmitted from the output voltage detection circuit, a level of a
current to flow through said switching element;a drain current detection
circuit which outputs a voltage according to a current flowing through
said switching element;a comparator which generates a signal that causes
said switching element to be turned off, when the voltage output by said
drain current detection circuit reaches one of the level determined by
said feedback control circuit and a reference voltage level; andan
off-period adjustment circuit which adjusts an off period based on a
control signal to be provided to said switching element and an output
signal provided from said oscillator, the off period being a period
during which said switching element is kept in an off state, andsaid
control circuit changes the switching oscillation frequency by changing
the off period of said switching element by causing said off-period
adjustment circuit to change a period indicated as an off period of said
switching element by a signal generated by said oscillator, according to
a time which the current flowing through said switching element has taken
to reach an overcurrent detection level after said switching element is
kept in the off state during the adjusted off period.

2. The semiconductor device according to claim 1,wherein a capacitor of
said off-period adjustment circuit is charged by a constant current
source during a period indicated as an on state of said switching element
by a gate signal which causes said switching element to operate, said
capacitor holds a voltage while the gate signal indicates an off state of
said switching element, a current source is generated from the voltage,
and a period indicated as an off state of said switching element by the
gate signal is changed using a current from said current source.

3. The semiconductor device according to claim 1,wherein said off-period
adjustment circuit outputs an electrical signal according to duration of
a period indicated as an on state of said switching element by a gate
signal which causes said switching element to operate and a value of a
residual current which is a current detected by said drain current
detection circuit when said switching element enters the on state,
andsaid oscillator is configured to, according to the electrical signal,
make a period indicated as the off state by the gate signal shorter as
the period indicated as the on state by the gate signal is longer and the
residual current is smaller, and makes a period indicated as the off
state by the gate signal longer as the period indicated as the on state
by the gate signal is shorter and the residual current is larger.

4. The semiconductor device according to claim 1, further comprisinga
high-current operation detection circuit which detects a high-current
operation performed when the current detected by said drain current
detection circuit is larger than a predetermined reference value,wherein
said oscillator is configured to fix, regardless of the electrical
signal, duration of a period indicated as the off state by the gate
signal in the case where said high-current operation detection circuit
detects the high-current operation.

5. The semiconductor device according to claim 1,wherein said
semiconductor device is controlled using a resistor in a circuit which
generates the current source in said off-period adjustment circuit, and
said resistor is provided outside of said semiconductor device so that
the off period is adjusted as necessary by replacing said resistor with
an other resistor.

6. A switching power supply apparatus comprising:the semiconductor device
according to claim 1;a switching element provided in said semiconductor
device;a transformer which transforms an input alternating current
voltage generated from an input direct current voltage into an output
alternating current voltage by switching operation on the input
alternating current voltage performed by said switching element; anda
smoothing circuit which converts the output alternating current voltage
into the output direct current voltage.

Description:

BACKGROUND OF THE INVENTION

[0001](1) Field of the Invention

[0002]The present invention relates to switching power supply apparatuses
and semiconductor devices which regulate output voltages to supply to
loads by switching at direct current voltages.

[0003](2) Description of the Related Art

[0004]Switching power supply apparatuses have been widely used as power
supply units of appliances for common household use, such as home
electrical products, for purposes such as improvement of power efficiency
by reducing power consumption. A switching power supply apparatus
includes a semiconductor device for switching power supply regulation
(hereinafter referred to as a regulation device) which regulates
(stabilizes) output voltages through switching operation of
semiconductors (switching elements such as transistors) (for example, see
Patent Reference 1: Japanese Unexamined Patent Application Publication
No. 2007-166810).

[0005]A conventional regulation device includes an oscillator 111 which
controls switching cycles of a switching element 102, a drain current
detection circuit 114 which detects drain current so that peak current of
the switching element 102 is controlled, a feedback control circuit 118
which detects states of output voltage via a photocoupler or the like so
that an on period of the switching element 102 is controlled.

[0006]The drain current detection circuit 114 detects a value of current
flowing through the switching element 102, which performs switching
operation at a cycle predetermined by the oscillator 111. Peak current of
the switching element 102 is controlled by a gate driver 113 based on an
output signal provided from the drain current detection circuit 114
through a NOR circuit 120 and an RS flip-flop 112. Furthermore, constant
voltage control is performed by providing PWM control through which on
periods of the switching element 102 is changed depending on a detected
state of the output voltage, so that voltage applied to the load is kept
constant.

[0007]The conventional regulation device is thus configured to keep output
voltage of the switching power supply apparatus constant by controlling a
drain current of the switching element.

[0008]However, there is a problem with a switching power supply apparatus
which includes the conventional regulation device is used.

[0009]In a switching power supply apparatus which includes the
conventional regulation device as shown in FIG. 1, switching operation of
the switching element 102 is stopped by the drain current detection
circuit 114 of the switching element 102 when current flowing through the
switching element 102 reaches a predetermined value. In this case, the
slope of the temporal change in the current flowing through the switching
element 102 is represented as Vin/L, where L is an L value of a
transformer (not shown) and Vin is an input voltage. In other word,
the higher the input voltage Vin is, the greater the slope is.

[0010]Operation of the drain current detection circuit 114 to stop the
operation of switching element 102 involves a certain delay time due to
delay times inherent in elements in the circuitry. The peak current of
the switching element 102 is thus represented as a product of Vin/L
and a current value after the certain delay time which has elapsed since
when the drain current detection circuit 114 detects drain current having
a current value referenced by the drain current detection circuit 114.

[0011]The slope of change in the current flowing through the switching
element 102 changes depending on input voltage as shown in FIG. 2.
Considering the delay time of the operation of stopping the switching of
the switching element 102, a high input voltage and a low input voltage
result in different peak drain currents, and thus maximum output power of
the switching power supply apparatus varies with the input voltage.

[0012]In view of this, higher input voltages used in some countries cause
high drain voltages, resulting in problems such as increase in ripple
voltage at output terminals and increase in loss due to on resistance of
switching elements.

[0013]In addition, since the change in ripple voltage causes change in
output power to loads and loss due to on-resistance increases, higher
voltages lead to lower efficiency of switching power supplies and
increase in self-heating of switching elements.

[0014]It is thus necessary that circuit constants of regulation devices
are accordingly adjusted to specifications of switching power supply
apparatuses used with input voltages different worldwide, in order to
make currents to loads uniform against variations in characteristics of
switching power supply apparatuses of the same circuit configuration used
worldwide. This makes reduction in cost of switching power supply
apparatuses difficult.

SUMMARY OF THE INVENTION

[0015]The present invention, conceived to address the problem with the
conventional switching power supply apparatus, provides a semiconductor
device for switching power supply regulation and a switching power supply
apparatus which includes the semiconductor device, where the
semiconductor device makes currents to loads uniform without providing
different circuit constants according to a range of input voltages;
thereby allowing reduction in cost of regulation devices.

[0016]In order to solve this problem, the semiconductor device according
to the present invention includes a control circuit which controls a
switching power supply apparatus that converts an input direct current
voltage into a regulated output direct current voltage, wherein the
switching power supply apparatus includes an output voltage detection
circuit which detects a change in the output direct current voltage and
transmits, to the control circuit, a feedback signal so that the control
circuit controls switching operation which a switching element performs
by supplying and blocking an input direct current voltage, the control
circuit includes:

an oscillator configured to determine a switching oscillation frequency of
the switching element; a feedback control circuit which determines,
according to the feedback signal transmitted from the output voltage
detection circuit, a level of a current to flow through the switching
element; a drain current detection circuit which outputs a voltage
according to a current flowing through the switching element; a
comparator which generates a signal that causes the switching element to
be turned off, when the voltage output by the drain current detection
circuit reaches one of the level determined by the feedback control
circuit and a reference voltage level; and an off-period adjustment
circuit which adjusts an off period based on a control signal to be
provided to the switching element and an output signal provided from the
oscillator, the off period being a period during which the switching
element is kept in an off state, and the control circuit changes the
switching oscillation frequency by changing the off period of the
switching element by causing the off-period adjustment circuit to change
a period indicated as an off period of the switching element by a signal
generated by the oscillator, according to a time which the current
flowing through the switching element has taken to reach an overcurrent
detection level after the switching element is kept in the off state
during the adjusted off period.

[0017]Furthermore, in the semiconductor device, a capacitor of the
off-period adjustment circuit may be charged by a constant current source
during a period indicated as an on state of the switching element by a
gate signal which causes the switching element to operate, the capacitor
may hold a voltage while the gate signal indicates an off state of the
switching element, a current source may be generated from the voltage,
and a period indicated as an off state of the switching element by the
gate signal may be changed using a current from the current source.

[0018]Furthermore, the off-period adjustment circuit may output an
electrical signal according to duration of a period indicated as an on
state of the switching element by a gate signal which causes the
switching element to operate and a value of a residual current which is a
current detected by the drain current detection circuit when the
switching element enters the on state, and the oscillator may be
configured to, according to the electrical signal, make a period
indicated as the off state by the gate signal shorter as the period
indicated as the on state by the gate signal is longer and the residual
current is smaller, and makes a period indicated as the off state by the
gate signal longer as the period indicated as the on state by the gate
signal is shorter and the residual current is larger.

[0019]Furthermore, the semiconductor device may further include a
high-current operation detection circuit which detects a high-current
operation performed when the current detected by the drain current
detection circuit is larger than a predetermined reference value, wherein
the oscillator is configured to fix, regardless of the electrical signal,
duration of a period indicated as the off state by the gate signal in the
case where the high-current operation detection circuit detects the
high-current operation.

[0020]Furthermore, the present invention may be implemented not only as
such a semiconductor device but also as a switching power supply
apparatus which includes the semiconductor device, a switching element
which is provided in the semiconductor device and generates input
alternating current voltage by switching input direct current voltage, a
transformer which transforms the input alternating current voltage
generated by the switching element into output alternating current
voltage, and a smoothing circuit which converts the output alternating
current voltage to output direct current voltage.

[0021]The present invention makes it possible to provide a power supply
which has output characteristics uniform against input voltages used
worldwide by regulating oscillation frequency of a switching element.

[0022]Regulation devices thereby provide uniform currents to loads without
being provided different circuit constants according to a range of input
voltages, and thus cost of switching power supply apparatuses is reduced.

Further Information About Technical Background to This Application

[0023]The disclosure of Japanese Patent Application No. 2009-158293 filed
on Jul. 2, 2009 including specification, drawings and claims is
incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]These and other objects, advantages and features of the invention
will become apparent from the following description thereof taken in
conjunction with the accompanying drawings that illustrate a specific
embodiment of the invention. In the Drawings:

[0025]FIG. 1 is a circuit diagram which illustrates an exemplary
configuration of a conventional semiconductor device;

[0026]FIG. 2 is a drawing which illustrates currents flowing through a
switching element for different input voltages of the conventional
semiconductor device;

[0027]FIG. 3 is a circuit diagram which illustrates an exemplary
configuration of a switching power supply apparatus including a
semiconductor device according to Embodiment 1 of the present invention;

[0028]FIG. 4 is a schematic view which illustrates overcurrent detection
levels for feedback currents in the semiconductor device according to
Embodiment 1;

[0029]FIG. 5 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit and an oscillator
included in the semiconductor device according to Embodiment 1;

[0030]FIG. 6 is a drawing which illustrates currents flowing through a
switching element for different input voltages of the semiconductor
device according to Embodiment 1;

[0031]FIG. 7 is a functional block diagram which illustrates an exemplary
configuration of a switching power supply apparatus and a semiconductor
device for controlling the same according to Embodiment 2 of the present
invention;

[0032]FIG. 8 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit and an oscillator
included in the semiconductor device according to Embodiment 2;

[0033]FIG. 9 is a drawing which illustrates currents flowing through a
switching element for different input voltages of the semiconductor
device according to Embodiment 2;

[0034]FIG. 10 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit and an oscillator
included in a semiconductor device according to Embodiment 3;

[0035]FIG. 11 is a drawing which illustrates currents flowing through a
switching element for different input voltages of the semiconductor
device according to Embodiment 3;

[0036]FIG. 12 is a circuit diagram which shows an exemplary configuration
of a switching power supply apparatus including a semiconductor device
according to Embodiment 4 of the present invention; and

[0037]FIG. 13 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit and an oscillator
included in the semiconductor device according to Embodiment 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038]The following specifically describes a switching power supply
apparatus and a semiconductor device for control of the switching power
supply apparatus of the according to embodiments of the present invention
with reference to drawings.

Embodiment 1

[0039]The following describes a switching power supply apparatus 100 and a
semiconductor device 4 used for control of the switching power supply
apparatus 100 according to Embodiment 1 of the present invention.

[0040]FIG. 3 is a functional block diagram which illustrates an exemplary
configuration of the switching power supply apparatus 100 according to
Embodiment 1.

[0041]In the switching power supply apparatus 100, a transformer 1, which
outputs an alternating current voltage resulting from waveform conversion
from a direct-current input voltage Vin performed through switching
operation of the switching element 2, includes a primary winding 1a and a
secondary winding 1b. The primary winding 1a and the secondary winding 1b
are opposite in polarity. The secondary winding 1b provides alternating
current voltage. The alternating current is converted into an output
direct current voltage by a smoothing circuit 7 and then supplied to a
load 8. The smoothing circuit 7 includes a diode 7a and a capacitor 7b.
The switching power supply apparatus 100 is a flyback switching power
supply apparatus. The switching element 2 is connected to the primary
winding 1a in series. Switching on and off the switching element 2 is
controlled according to an output signal provided from a control circuit
3 to a control electrode of the switching element 2.

[0042]The semiconductor device 4 includes the control circuit 3 and the
switching element 2. The switching element 2, which may be a power
MOSFET, and the control circuit 3 are integrated on a single
semiconductor substrate.

[0043]A DRAIN terminal is a junction point between the primary winding 1a
of the transformer 1 and the switching element 2. That is, the DRAIN
terminal is a terminal connected to a drain of the switching element 2.

[0044]A GND terminal, which is a terminal to connect a source of the
switching element 2 and GND of the control circuit 3 to a ground level,
is connected to a terminal of a lower potential between two terminals to
which input voltages Vin are applied.

[0045]A VDD terminal, to which a capacitor 5 is connected, is a terminal
through which the capacitor 5 is charged by a current from a regulator 9
incorporated in the control circuit 3 in order to control power supply
voltage of the control circuit 3.

[0046]An FB terminal provides a feedback control circuit 18 of the control
circuit 3 with a feedback signal (for example, a current generated by a
phototransistor) provided from the output voltage detection circuit 6.
The feedback signal indicates a direct-current output voltage Vout.

[0047]The regulator 9 is connected to the DRAIN terminal of the switching
element 2, the VDD terminal, and the start-up shutdown circuit 10. The
regulator 9 supplies a current provided from the DRAIN terminal to the
capacitor 5 via the VDD terminal to increase an auxiliary power supply
voltage VDD when the input voltage Vin is applied to the DRAIN
terminal of the switching element 2 via the transformer 1. When voltage
at the VDD terminal increases to a start-up voltage, the regulator 9
stops supplying the current from the DRAIN terminal to the capacitor 5.
When the voltage at the VDD terminal decreases to the start-up voltage or
lower again, the regulator 9 starts supplying a current from the DRAIN
terminal to the VDD terminal to increase the voltage at the VDD terminal
again.

[0048]A start-up shutdown circuit 10 monitors the voltage at the VDD
terminal and controls start-up and shutdown of switching operation of the
switching element 2 (in other words, turns on and off the switching
element 2) depending on the magnitude of the voltage at the VDD terminal.

[0049]The feedback control circuit 18 determines an overcurrent detection
level which indicates an upper limit of the current flowing through the
switching element 2 according to the feedback signal provided from the
output voltage detection circuit 6 to the FB terminal of the control
circuit 3 so that the output voltage Vout indicated by the feedback
signal is stably maintained as shown in FIG. 4. The feedback control
circuit 18 then provides an output voltage which indicates the determined
overcurrent detection level to a negative input of a comparator 16.

[0050]With this configuration, the current flowing through the switching
element 2 decreases when the output voltage Vout increases under a
light load, and the current flowing through the switching element 2
increases when the output voltage Vout decreases under a heavy load.

[0051]A drain current detection circuit 14 relatively detects the drain
current flowing through the switching element 2 by detecting, for
example, an on voltage which is determined as a product of the drain
current flowing through the switching element 2 and an on resistance of
the switching element 2, and then provides a voltage signal in proportion
to the detected drain current to a positive input of the comparator 16.

[0052]The comparator 16 outputs a high-level signal when a voltage
indicating the drain current provided into the positive input is equal to
or larger than a lower one of a voltage of the output signal provided
from the feedback control circuit 18 into a first negative input and a
reference voltage 19 at a second negative input.

[0053]A turn-on blanking pulse generation circuit 15 outputs a blanking
pulse for a certain blanking time after the gate driver 13 provides a
high-level GATE signal to the switching element 2 in order to prevent the
drain current detection circuit 14 from false detection of, for example,
a capacitive current spike due to capacitance of the switching element 2.

[0054]An off-period adjustment circuit 17 receives an oscillation starting
signal from the oscillator 11 and an output signal from the drain current
detection circuit 14, and changes an off period of the oscillator 11
depending on on-period duration of switching converted into voltages in
order to adjust duration of an off period of the switching element 2. The
off-period adjustment circuit 17 and an exemplary circuit configuration
thereof will be detailed in description of operation below.

[0055]Once the semiconductor device 4 is turned on, an RS flip-flop 12
receives a pulse signal, that is, a CLOCK signal, at a high level from
the oscillator 11 into a set input S thereof, and thus an output Q rises
to a high level and the GATE signal of the gate driver 13 rises to a high
level, so that the switching element 2 enters an on state.

[0056]On the other hand, when the switching element 2 has entered the on
state and, after a turn-on blanking time, receives a current at an
overcurrent detection level, which has been determined according to the
feedback signal from the output voltage detection circuit 6, from the
feedback control circuit 18, the output signal from the comparator 16
rises to a high level and is provided to a reset input R of the RS
flip-flop 12 via an OR circuit 20. The output Q from the RS flip-flop 12
thus falls to a low level and the output of the gate driver 13 falls to a
low level, so that the switching element 2 enters an off state.

[0057]Alternatively, when the output of the comparator 16 remains at a low
level during a maximum on period in which a MAXDUTY signal of the
oscillator 11 is at a low level, the MAXDUTY signal of the oscillator 11
inverts to a high level after the maximum on period has elapsed. The
MAXDUTY signal at a high level is provided to the reset input R of the RS
flip-flop 12 via the OR circuit 20. The output Q of the RS flip-flop 12
thus falls to a low level and the output of the gate driver 13 falls to a
low level, so that the switching element 2 is forcibly put into an off
state regardless of the amount of the current flowing through the
switching element 2.

[0058]This is signal processing which causes switching on and off of the
switching element 2.

[0059]In the transformer 1, the input voltage Vin is
waveform-converted by the switching operation of the switching element 2,
and thus an alternating current voltage is induced in the secondary
winding 1b. The secondary winding 1b of the transformer 1 is connected to
an output voltage generation unit 7 which includes the diode 7a for
rectification and the capacitor 7b. The output voltage generation unit 7
rectifies and smoothes the induced alternating current voltage to
generate the direct-current output voltage Vout to be applied to the
load 8.

[0060]The output voltage detection circuit 6 includes, for example, an LED
and a zener diode. The output voltage detection circuit 6 detects a
voltage level of the output voltage Vout and outputs a feedback
signal which is necessary for the control circuit 3 to control switching
operation of the switching element 2 so that the output voltage Vout
is stable at a predetermined voltage.

[0061]Here, it is assumed that, in the switching power supply apparatus
100, the primary winding 1a of the transformer 1 for power conversion is
provided with the direct-current input voltage Vin which is prepared
from commercial alternating current power through rectification by a
rectifier such as a diode bridge and smoothing by an input capacitor.

[0062]The following describes operation of the switching power supply
apparatus 100 and the semiconductor device 4 used in the switching power
supply apparatus 100 shown in FIG. 3 and configured as described above.

[0063]The alternating current power sourced from the commercial power
supply is rectified by the rectifier such as a diode bridge and smoothed
by an input capacitor, which are not shown in the drawings, and then
converted into the direct-current input voltage Vin. The input
voltage Vin is applied to the DRAIN terminal via the primary winding
1a of the transformer 1. A start-up charge current then flows from the
DRAIN terminal, via the regulator 9 of the control circuit 3, to the
capacitor 5 connected to the VDD terminal. When the voltage at the VDD
terminal of the control circuit 3 increased by the charge current reaches
the start-up voltage set in the start-up shutdown circuit 10, control of
the switching operation of the switching element 2 is started.

[0064]When the switching element 2 is turned on, a current flows through
the switching element 2, and a voltage depending on the magnitude of the
current flowing through the switching element 2 is provided from an
output of the drain current detection circuit 14 and into the positive
input of the comparator 16. In addition, an output voltage according to
the feedback signal from the output voltage detection circuit 6 is
provided from the feedback control circuit 18 into the first negative
input of the comparator 16, and a reference voltage 19 is provided into
the second negative input of the comparator 16.

[0065]When the output voltage of the drain current detection circuit 14
rises to be equal to or higher than a lower one of the output voltage of
the feedback control circuit 18 and the reference voltage 19, a
high-level signal is provided from the comparator 16 into the reset input
R of the RS flip-flop 12, so that and the switching element 2 is turned
off.

[0066]There is a delay time between when the output voltage of the drain
current detection circuit 14 rises to be equal to or higher than one of
the output voltage of the feedback control circuit 18 and the reference
voltage 19 and when the switching element 2 is turned off.

[0067]When the switching element 2 is turned off, energy stored in the
primary winding 1a of the transformer 1 during the on period of the
switching element 2 is transferred to the secondary winding 1b.

[0068]The output voltage Vout increases through repetition of the
switching operation above. When the output voltage Vout becomes
equal to or higher than a voltage set in the output voltage detection
circuit 6, the output voltage of the feedback control circuit 18
decreases depending on the magnitude of the feedback current from the FB
terminal of the control circuit 3 provided as a feedback signal from the
output voltage detection circuit 6, and thus the voltage at the negative
input of the comparator 16 decreases, so that the current flowing through
the switching element 2 decreases. This is how the state of on-duty of
the switching element 2 becomes appropriate.

[0069]In other words, the period of time for which the current flows
through the switching element 2 is shorter when the load is light with a
small current supply to the load 8, and longer when the load is heavy.

[0070]The following describes the off-period adjustment circuit 17 and the
oscillator 11 in detail.

[0071]FIG. 5 is a circuit diagram which illustrates an exemplary
configuration of the off-period adjustment circuit 17 and the oscillator
11.

[0072]In FIG. 5, the off-period adjustment circuit 17 and the oscillator
11 include constant current sources 21, 22, 23, 24, and 54, p-type
MOSFETs 25, 26, 27, 28, and 29, n-type MOSFETs 30, 31, 32, 33, 34, 35,
52, and 53, capacitors 36 and 37, a resistor 38, an NPN bipolar
transistor 39, a comparator 40, an inverter circuit 41, a reference
voltage source 42, and a CLOCK signal generator 43. The pair of the
p-type MOSFETs 26 and 27, the pair of the n-type MOSFET 31 and 32, the
pair of the n-type MOSFET 33 and 52, and the pair of the n-type MOSFETs
53 and 34 each form a current-mirror circuit.

[0073]When the output of the comparator 40 is at a low level, the p-type
MOSFET 29 turns on, a current flows from the constant current source 24
into the capacitor 37 to charge the capacitor 37, and then the voltage
across the capacitor 37 is provided into a positive input of the
comparator 40. The reference voltage source 42 outputs a lower reference
voltage and a higher reference voltage for a high-level output and a
low-level output of the comparator 40, respectively. This produces a
hysteresis of the output of the comparator 40.

[0074]Until the voltage across the capacitor 37 reaches the higher
reference voltage of the reference voltage source 42, the output of the
comparator 40 is at the low level, and thus the MAXDUTY signal is at the
low level indicating the maximum on period of the switching element 2.

[0075]When the output of the comparator 40 falls to the low level, the
CLOCK signal generator 43 differentiates the output of the comparator 40
at an edge of the falling so as to output a CLOCK signal at a high level
for a short period.

[0076]A CLOCK signal at a high level is provided into the set input S of
the RS flip-flop 12, and then the output Q from the RS flip-flop 12 rises
to a high level. This causes the GATE signal, which is the output of the
gate driver 13, to rise to a high level, so that the switching element 2
is turned on.

[0077]The high-level CLOCK signal is also provided into the gate of the
n-type MOSFET 30, and then the n-type MOSFET 30 is turned on to cause the
voltage of the capacitor 36 to fall to the GND voltage.

[0078]When the GATE signal is at the high level, a low-level signal is
provided into the gate of the p-type MOSFET 25 through the inverter
circuit 41. This turns on the p-type MOSFET 25, and thus a current is
provided from the constant current source 21 to the capacitor 36.

[0079]The base voltage of the NPN bipolar transistor 39 is equal to the
sum of the voltage of the capacitor 36 and a Vth voltage of the
p-type MOSFET 28. A current flowing through the NPN bipolar transistor 39
is determined by a resistance of the resistor 38 and a voltage obtained
by subtracting a VBE voltage of the bipolar transistor 39 from the
base voltage.

[0080]A current flowing through the n-type MOSFET 32 is determined by the
current flowing through the NPN bipolar transistor 39, the mirror circuit
made up of the p-type MOSFETs 26 and 27 and the mirror circuit made up of
the n-type MOSFETs 31 and 32. A current flowing through the n-type MOSFET
33 is decreased by passing a current from the constant current source 23
to the n-type MOSFET 32, and a current flowing through the n-type MOSFET
53 is determined by passing a current from the constant current source 54
to the n-type MOSFET 52 so that the current flowing through the n-type
MOSFET 53 increases as the current flowing through the n-type MOSFET 32
increases.

[0081]Next, the output of the comparator 16 rises to a high level when the
output of the drain current detection circuit 14 rises to be equal to or
higher than one of a voltage of the feedback signal from the output
voltage detection circuit 6 and the reference voltage 19. A high level
signal is then provided into the reset input R of the RS flip-flop 12
causing the output Q of the RS flip-flop 12 to fall to a low level, so
that the output of the gate driver 13 falls to a low level.

[0082]This turns off the p-type MOSFET 25 and stops a charging current
from the constant current source 21 to the capacitor 36. The capacitor 36
holds a voltage which has been increased while the GATE signal is at a
high level (that is, during an on period of the switching element 2), and
thus a current value of the n-type MOSFET 34 while the n-type MOSFET 35
is on is determined.

[0083]When the voltage across the capacitor 37 reaches the higher
reference voltage of the reference voltage source 42, the output voltage
of the comparator 40 changes to a high level and the MAXDUTY signal
changes to a high level indicating an off period. This turns off the
p-type MOSFET 29, turns on the n-type MOSFET 35, and thus the voltage of
the reference voltage source 42 changes to the lower reference voltage.

[0084]The voltage across the capacitor 37 decreases at a rate according to
a value of the current flowing through the n-type MOSFET 34. When the
voltage across the capacitor 37 becomes equal to or lower than the lower
reference voltage, the output from the comparator 40 inverts to a low
level, and the MAXDUTY signal returns to the low level indicating the
maximum on period of the switching element 2.

[0085]Thus, in the switching power supply apparatus 100, a period of each
cycle (in other words, a switching frequency) of the switching element 2
is adjusted by changing duration of an off period which follows each on
operation of the switching element 2 so as to make the voltage across the
capacitor 36 equal to the GND voltage using the CLOCK signal provided
from the oscillator 11.

[0086]The following describes effects of the operation above.

[0087]When the switching element 2 is turned on, charging the capacitor 36
is started. The voltage across the capacitor 36 increases while the
switching element 2 is on. After the switching element 2 turns off, the
capacitor 36 holds the increased voltage. When the output from the
comparator 40 is at a high level indicating an off-period, and the n-type
MOSFET 35 is turned on, the current flowing through the n-type MOSFET 34
increases or decreases depending on the voltage held by the capacitor 36,
so that the duration of the off period changes which is indicated by the
output of the comparator 40 at the high level.

[0088]This is expressed by the equations below.

[0089]The voltage across the capacitor 36, V1, is expressed by EQ. 1
in relation to a period from when the switching element 2 is turned on to
when the switching element 2 is turned off:

V1=ton×Iconst1/C1 (EQ. 1),

[0090]where [0091]C1 is a capacity of the capacitor 36;
[0092]ton is a period from when the switching element 2 is turned on
to when the switching element 2 is turned off; and [0093]Iconst1 is
a current flowing from the constant current source 21.

[0094]The value of V1 is maintained until the CLOCK signal is output
after the switching element 2 is turned off.

[0095]The current I1 which flows through the p-type MOSFET 26 is
expressed by EQ. 2:

I1=(V1+Vpth+VBE)/R (EQ. 2),

[0096]where [0097]Vpth is a threshold of the p-type MOSFET 26;
[0098]VBE is a voltage between base and emitter terminals of the NPN
bipolar transistor 39; and [0099]R is a resistance of the resistor 38.

[0100]The current I2 which flows through the n-type MOSFET 34 is
expressed by EQ. 3:

I2=(Iconst3-((Iconst2-I1×n1×n2).-
times.n3))×n4 (EQ. 3),

[0101]where [0102]Iconst2 is a current flowing from the constant
current source 23; [0103]Iconst3 is a current flowing from the
constant current source 54; [0104]n1 is a mirror ratio of the
current-mirror circuit made up of the p-type MOSFETs 26 and 27;
[0105]n2 is a mirror ratio of the current-mirror circuit made up of
the n-type MOSFETs 31 and 32; [0106]n3 is a mirror ratio of the
current-mirror circuit made up of the n-type MOSFETs 33 and 52; and
[0107]n4 is a mirror ratio of the current-mirror circuit made up of
the n-type MOSFETs 53 and 34.

[0108]Thus, the voltage V1 of the capacitor 36 increases as the on
period ton of the switching element 2 becomes longer, so that the
current I1 of the p-type MOSFET 26 increases and the current I2
of the n-type MOSFET 34 thus increases. Conversely, the current I2
of the n-type MOSFET 34 decreases as the on period ton of the
switching element 2 becomes shorter.

[0109]The duration PW1 of the maximum on period, which is indicated
by the MAXDUTY signal at a low level, is expressed by EQ. 4:

PW1=ΔV×C2/Iconst4 (EQ. 4),

[0110]where [0111]ΔV is a difference between the two reference
voltages of the reference voltage source 42; [0112]C2 is a capacity
of the capacitor 37; and [0113]Iconst4 is a current flowing from the
constant current source 24.

[0114]The duration PW2 of the off period, which is indicated by the
MAXDUTY signal at a high level, is expressed by EQ. 5:

PW2=ΔV×C2/I2 (EQ. 5),

[0115]where [0116]I2 is a current flowing through the n-type MOSFET
34.

[0117]Thus, the off period PW2 becomes shorter as the on period
ton of the switching element 2 becomes longer. Conversely, the
off-period PW2 becomes longer as the on period ton becomes
shorter.

[0118]As shown in (A) and (B) of FIG. 6, the on period of the switching
element 2 is shorter and thus the off period is longer when the input
voltage is high than when the input voltage is low. As a result, the
switching frequency decreases accordingly.

[0119]The energy P applied to the load is expressed by EQ. 6:

P=1/2×π×L×IDp2×f (EQ. 6),

[0120]where [0121]π is efficiency; [0122]L is an inductance of the
transformer 1; [0123]IDp is a peak current of the switching element
2; and [0124]f is a switching frequency of the switching element 2.

[0125]As described in the Description of the Related Art above, since the
peak current of the switching element 2 is larger when a switching power
supply apparatus is operating at a high input voltage than at a low input
voltage, energy supplied to the load may be too large when no adjustment
is made for the off period.

[0126]In the switching power supply apparatus 100 operating at a high
input voltage, the switching frequency is lowered with an extended
minimum off period by taking advantage of a shorter on period ton of
the switching element 2 than when operating at a low input voltage.
Change in the energy P supplied to the load is thus reduced even when
there is a change in the input voltage.

[0127]The switching power supply apparatus 100 performs such operation
without changing a circuit constant. A switching power supply apparatus
is thus provided in which change in current to a load depending on input
voltage is small, advantageously allowing reduction in cost.

Embodiment 2

[0128]The following describes a switching power supply apparatus 200 and a
semiconductor device 204 used for control of the switching power supply
apparatus 200 according to Embodiment 2 of the present invention.

[0129]FIG. 7 is a functional block diagram which illustrates an exemplary
configuration of the switching power supply apparatus 200 according to
Embodiment 2.

[0130]FIG. 8 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit 217 and the oscillator
11 included in the semiconductor device 204.

[0131]The semiconductor device 204 differs from the semiconductor device 4
included in the switching power supply apparatus 100 according to
Embodiment 1 in that the output of the drain current detection circuit 14
and an output of the turn-on blanking pulse generation circuit 15 are
connected to the off-period adjustment circuit 217, and in that a
variable current source 47 is connected in parallel with the constant
current source 21 in the off-period adjustment circuit 217.

[0132]Since a structure and operation of the switching power supply
apparatus 200 are generally the same as those described in Embodiment 1,
the following describes only differences of Embodiment 2 from Embodiment
1.

[0133]In FIG. 8, the drain current detection circuit 14 is a shunt circuit
including resistors 44 and 45. In comparison with the off-period
adjustment circuit 17, the off-period adjustment circuit 217 additionally
includes a switch 46 and a variable current source 47. The switch 46 has
one end connected to the output of the drain current detection circuit 14
and the other end connected to the variable current source 47, and is
turned on and off by blanking pulses provided from the turn-on blanking
pulse generation circuit 15.

[0134]The turn-on blanking pulse generation circuit 15, for example,
differentiates a MAXDUTY signal at a falling edge, so as to output a
blanking pulse for a certain period of time immediately after the MAXDUTY
signal indicates a maximum on period. The switch 46 is turned on by the
blanking pulse, and then a current flowing through the variable current
source 47 is determined by an output voltage of the drain current
detection circuit 14 while the switch 46 is kept on by the blanking
pulse.

[0135]The output voltage of the drain current detection circuit 14 while
the blanking pulse is generated indicates a current residing in the
primary winding 1a of the transformer 1 when the switching element 2 is
turned on (the current is hereinafter referred to as a residual current).
The variable current source 47 outputs, depending on the output voltage
of the drain current detection circuit 14 when the blanking pulse is
generated, a smaller current as the residual current is larger, and a
larger current as the residual current is smaller.

[0136]The capacitor 36 is charged by a current from the variable current
source 47 and a current from the constant current source 21. In
comparison with Embodiment 1, the voltage across the capacitor 36 thus
increases with increase in the current from the variable current source
47. The current flowing through the n-type MOSFET 34 changes through the
operation which is similar to the operation in Embodiment 1 depending on
the voltage across the capacitor 36 including the increase according to
the current from the variable current source 47.

[0137]This is expressed by the equations below.

[0138]The voltage V1 across the capacitor 36 is expressed by EQ. 7 in
relation to on period ton of the switching element 2:

V1=ton×(Iconst1+Ivar)/C1 (EQ. 7),

[0139]where [0140]C1 is a capacity of the capacitor 36;
[0141]ton is a period from when the switching element 2 is turned on
to when the switching element 2 is turned off; [0142]Iconst1 is a
current flowing from the constant current source 21; and [0143]Ivar
is a current flowing through the variable current source 47.

[0144]The current I2 of the n-type MOSFET 34 and the duration
PW2 of the off period which is indicated by the MAXDUTY signal at a
high level are determined by EQ2, EQ, 3, and EQ. 5 as in Embodiment 1,
using the voltage V1 across the capacitor 36 expressed by EQ. 7.

[0145]In the switching power supply apparatus 200, the duration of the off
period PW2 changes depending not only on the on period ton of
the switching element 2 but also on the current flowing through the
switching element 2 when a blanking pulse is generated.

[0146]As shown in (A) and (B) of FIG. 9, if the switching element 2 is
turned on when there is a current residing in the primary winding 1a of
the transformer 1 (in other words, a residual current is present), the
switching element 2 generates a drain current having the same value as
the residual current Is. Energy P to be applied to the load is
expressed by EQ. 8.

P=1/2×π×L×(IDp2-Is2)×f
(EQ. 8),

[0147]where [0148]π is efficiency; [0149]L is an inductance of the
transformer 1; [0150]IDp is a peak current of the switching element
2; [0151]Is is a residual current when the switching element 2 is
turned on; and [0152]f is a switching frequency of the switching element
2.

[0153]EQ. 8 shows that the energy applied to the load decreases when the
residual current Is is large.

[0154]In the switching power supply apparatus 200, the current to be used
for charging the capacitor 37 is changed to account for not only the on
period ton of the switching element 2 but also the residual current
Is at the time when the switching element 2 is turned on. The
residual current Is is thus reduced by extending the off period of
the switching element 2 in the case where the energy supplied to the load
decreases because of the largeness of the residual current Is, so
that reduction in energy to be supplied is compensated.

[0155]The amount of energy to be supplied to the load thus is kept
adequate to account for not only the input voltage but also the residual
current Is at the time of the turning-on.

Embodiment 3

[0156]The following describes a switching power supply apparatus 300 and a
semiconductor device 303 used for control of the switching power supply
apparatus 300 according to Embodiment 3 of the present invention.

[0157]FIG. 10 is a functional block diagram which illustrates an exemplary
configuration of the switching power supply apparatus 300 according to
Embodiment 3.

[0158]FIG. 11 is a circuit diagram which illustrates an exemplary
configuration of the off-period adjustment circuit 17 and an oscillator
311 included in the semiconductor device 304.

[0159]The switching power supply apparatus 300 differs from the switching
power supply apparatus 100 according to Embodiment 1 in that the
semiconductor device 303 further includes a high-current operation
detection circuit 51 which includes a comparator 48 and an RS flip-flop
49, and an oscillator 311 which further includes an n-type MOSFET 50.

[0160]Since a structure and operation of the switching power supply
apparatus 300 are generally the same as those described in Embodiment 1,
the following describes only differences of Embodiment 3 from Embodiment
1.

[0161]The drain current detection circuit 14 has an output connected to
the positive input of the comparator 48. The reference voltage 19 is
connected to one of the negative input of the comparator 48. The output
of the comparator 48 is connected to the set input S of the RS flip-flop
49. The CLOCK signal is provided from the oscillator 311 to the reset
input R of the RS flip-flop 49. The output Q of the RS flip-flop 49 is
connected to a gate terminal of the n-type MOSFET 50 included in the
oscillator 311.

[0162]When an output voltage of the drain current detection circuit 14
becomes higher than the reference voltage 19 while the switching element
2 is on, the output signal of the comparator 48 rises to a high level,
and the output Q of the RS flip-flop 49 is set to a high level. The
high-level output Q of the RS flip-flop 49 remains at the high level
until the RS flip-flop 49 is reset by a next CLOCK signal even while the
MAXDUTY signal indicates an off period, and turns on the n-type MOSFET 50
included in the oscillator 311.

[0163]When the output voltage of the drain current detection circuit 14 is
lower than the reference voltage 19, that is, a feedback operation is
performed, the output of the comparator 48 is at a low level, and thus
the output Q of the RS flip-flop 49 decreases to a low level. This keeps
the n-type MOSFET 50 included in the oscillator 311 off.

[0164]The off period of the switching element 2 is thus changed only while
the switching element 2 is switching at a drain current determined by the
reference voltage 19, so that deficiency in power supply due to change in
frequency of the switching element 2 while a feedback operation is
prevented by causing the operation of changing the off period of the
switching element 2 to be performed only at a maximum power.

[0165]Although the switching element 2 and the control circuit 3 described
above are provided on a single substrate, the control circuit 3 and the
switching element 2 may be provided on different substrates.

Embodiment 4

[0166]The following describes a switching power supply apparatus 400 and a
semiconductor device 403 used for regulation of the switching power
supply apparatus 400 according to Embodiment 4 of the present invention.

[0167]FIG. 12 is a functional block diagram which illustrates an exemplary
configuration of the switching power supply apparatus 400 according to
Embodiment 4.

[0168]FIG. 13 is a circuit diagram which illustrates an exemplary
configuration of an off-period adjustment circuit 417 and the oscillator
11 included in the semiconductor device 404.

[0169]Since a structure and operation of the switching power supply
apparatus 400 are generally the same as those described in Embodiment 1,
the following describes only differences of Embodiment 4 from Embodiment
1.

[0170]The switching power supply apparatus 400 differs from the switching
power supply apparatus 100 according to Embodiment 1 in that the
off-period adjustment circuit 417 included in the switching power supply
apparatus 400 is not provided with the resistor 38 of the off-period
adjustment circuit 17 (see FIG. 5), and in that a resistor 55 is
connected externally to the semiconductor device 404.

[0171]The emitter terminal of the NPN bipolar transistor 39 included in
the off-period adjustment circuit 417 is connected to the resistor 55
outside of the semiconductor device 404.

[0172]This configuration allows adjusting duration of an off period of the
switching element 2 as necessary by changing resistance of the resistor
55.

[0173]Although the switching element 2 and the control circuit 3 are
provided on a single substrate in this description, the control circuit 3
and the switching element 2 may be provided on different substrates.

[0174]Furthermore, although the switching power supply apparatus is
described as an insulated power-supply circuit in which a transformer is
used as a converter, the switching power supply apparatus may be a
non-insulated power supply apparatus in which a coil is used as a
converter.

[0175]These embodiments are described as examples of the present
invention, and any circuit configured to perform the operation according
to the present invention is included within the scope of the present
invention.

[0176]Although only some exemplary embodiments of this invention have been
described in detail above, those skilled in the art will readily
appreciate that many modifications are possible in the exemplary
embodiments without materially departing from the novel teachings and
advantages of this invention. Accordingly, all such modifications are
intended to be included within the scope of this invention.

INDUSTRIAL APPLICABILITY

[0177]Among switching power supply apparatuses according to the present
invention or semiconductor devices used therein, currents to loads are
made uniform without providing different circuit constants according to a
range of input voltages, thus cost of switching power supply apparatuses
is can be reduced. The present invention is thus effectively applicable
to switching power supply apparatuses such as an AC-AC converter and a
DC-DC converter.